Synthetic corundum crystals and process for making same



J. N. BURDICK ET AL 2,690,062

SYNTHETIC CORUNDUM CRYSTALS AND PROCESS FOR MAKING SAME 7 Sept. 28, 1954 'Filed Dec. 21, 1949 INVENTORS JOHN N. BURDICK ROBERT A.JONES ATTORNEY Patented Sept. 28, 1954 SYNTHETIC CORUNDUM CRYSTALS AND PROCESS FOR MAKING SAME John N. Burdick, Kenmore, and Robert A. Jones, Buffalo, N. Y., assignors, by mesne assignments, to Union Carbide and Carbon Corporation, a corporation of New York Application December 21, 1949, Serial No. 134,182

21 Claims. I

The present invention relates to a novel process for producing synthetic corundum crystals containing small amounts of titanium compound, and to the novel corundum crystals as articles of manufacture. The invention is particularly concerned with the provision of improved synthetic star sapphires and star rubies.

When a synthetic corundum monocrystal containing a compound of titanium is grown as a generally cylindrical boule by the well-known Verneuil process (as disclosed in United States Patent No. 1,004,505) from alumina powder having a small quantity of titania powder mixed therewith (with or without small quantities of coloring material such as chromium oxide or iron oxide), and stable thermal conditions are maintained, the titania concentrates in a zone adjacent the periphery of the crystal whilethe central portion of the crystal remains substantially free from titania. This is particularly detrimental when growing blue sapphire crystals because the blue color, which is due at least in part to titania, is concentrated near the surface of the crystal while the center remains nearly colorless. In ruby, however, the red color extends throughout the crystal.

It has recently been found that when a synthetic ruby or sapphire crystal is grown as a boule under stable thermal conditions from a powder containing a small quantity of titania as an essential ingredient, for example 0.1% to 0.3% of titania, and then is heat treated at a temperature between 1100 and 1500 0., a titanium compound, probably titania, precipitates out of solid solution as a cloudy silky precipitate, and the crystal then exhibits asterism, Synthetic star sapphires and star rubies are cut cabochon from such asteriated crystal boules in a conventional manner so that the tangent to the convex crown of the gemstone at its apex is normal to the crystallographic c-axis (which is thus parallel to the geometric axis of the stone), that the crown includes the asteriated peripheral zone, and that the substantially flat base of the gemstone is normal to the c-axis. The resulting gemstone exhibits a well-defined six-rayed star when viewed in reflected light- Due to the geometry of the boule and the concentration of titaniain a peripheral zone it'has been customary to cut cabochons from boules having a 90 c-axis orientation so as to produce symmetrical substantially complete stars. In such a cabochon, areas of the crown cut from the peripheral titania-bearing zone exhibit asterism, but areas on two opposite sides adjacent the base of the cabochon are often cut from the interior of the boule below the peripheral titania-bearing zone and thus may not exhibit asterism. As a result, the rays of the star which extend down these two opposite sides are slightly shorter than the other rays, which extend all the way to the base. Furthermore, in a synthetic blue star sapphire, the two opposite areas referred to above also are substantial- 1y free from blue color, so that when critically examined a blue synthetic star sapphire may display a broad blue asteriated band extending across the top of the cabochon, and two opposite ends of the cabochon may be substantially free from both color and asterism.

Additionally, the color of blue synthetic star sapphires heretofore has not been esthetically the most desirable, being a dark gray-blue instead of the desired good bright medium-dark blue. According to the Munsell system of color notation the chroma has been only about 4, whereas a chroma of between 8 and 10 is desirable for the same color hue and value.

In accordance with the present invention, there is provided a novel improvement on the Verneuil process for growing a synthetic corundum monocrystal containing titania whereby the titania appears in its coloring and asteriating effects ostensibly as though it were distributed homogeneously throughout the crystal from surface to center, even though such homogeneity is not actually achieved. This apparent homogeneity is obtained by passing powdered constituents of the crystal comprising a mixture of powdered alumina containing a small quantity of titania as an essential ingredient, desirably 0.1% to 0.3% of titania by weight, through an oxy-fuel gas flame to fuse the constituents; accumulating and crystallizing the fused constituents on a support aligned axially with the flame to form an axially lengthening crystal of increasing size; and, during the accumulating and crystallizing, maintaining fluctuating thermal conditions around the growing crystal, thereby causing it to grow as a series of thin longitudinally distributed upwardly convex partially spherical integrated transverse layers, alternate ones of said layers having titania distributed in solid solution therein across the full width thereof and each of the intermediate layers having titania dissolved therein in an annular zone adjacent the periphery thereof while the central portion thereof remains substantially free of tiania. Eluctuating thermal conditions advantageously are maintained by alternately increasing and decreasing the rate of heat supply to the growing crystal as by alternately increasing and decreasing the heating intensity of the flame at frequent intervals, but it is to be understood that other specific procedures for maintaining fluctuating thermal conditions will be apparent to those skilled in the art.

As is well known, synthetic ruby and sapphire boules in the as-grown condition must be split vertically to relieve internal stresses before they can be further worked. Such splitting is obviated when the whole boule, as grown by the above procedure, is annealed at a temperature of about 1950 C.

Still further in accordance with the invention, asteriated synthetic corundum crystals are provided which ostensibly (but not actually) are asteriated homogeneously throughout their masses, by heat treating at a temperature between 1100 and 1500 C. the annealed or unannealed novel crystals grown in accordance with the above procedure. A compound of titanium, probably titanium dioxide in the form of rutile crystallites, then precipitates from solid solution across the full width of alternate thin layers, but only adjacent the periphery of the intermediate thin layers, as a result of the novel titania distribution described above.

When a gem stone is cut cabochon in the conventional manner from the asteriated product described above in such a way that the convex crown of the cabochon is arranged symmetrically with respect to a geometric axis and the c-axis of the crystal is parallel to the geometric axis, it is found that all six rays of the star extend equally down the sides of the gem stone to its base. Moreover, in blue sapphire, the most desirable bright blue color having a chroma of between 8 and 10 by the Munsell system is obtained.

These star stones sometimes may exhibit a banded appearance when the cabochon is cut from a boule having a c-axis orientation much greater than zero degrees, due to the fact that the crown then cuts sharply across the several layers, alternate ones of which are not asteriated over part of their width. Therefore, in order to insure seeming homogeneity in the appearance of a blue sapphire or ruby cabochon, it is desirable to grow the synthetic corundum crystal boule with a c-axis orientation of about zero degree with respect to the longitudinal growth axis of the boule (a tolerance of about degrees being permissible for manufacturing convenience), and to cut the cabochon from the whole boule with a convex crown having approximately the same curvature as the layers in the boule. However, it is possible to cut commercially satisfactory star cabochons from ruby or blue boules and halfboules having orientations much greater than zero degrees, for example 90 degrees.

The principles of the invention will be described in detail hereinafter with reference to the accompanying drawings wherein:

Fig. l is a schematic vertical midsectional view, parts being in elevation, showing apparatus for growing a synthetic corundum crystal by the process of the invention;

Fig. 2 is a side elevational view showing a synthetic corundum boule;

Fig. 3 is a schematic magnified longitudinal midsectional view of the part of the boule between the planes aa and b-b in Fig. 2;

Fig. i is a schematic view similar to Fig. 3, showing how a gem stone is cut cabochon from a whole boule having a c-axis orientation of zero degrees;

Fig. 5 is a schematic view similar to Fig. 3, showing how a gem stone is cut cabochon from a whole boule having a c-axis orientation of degrees;

Fig. 6 is a perspective View looking down on the crown of an asteriated corundum gem stone cut cabochon; and

Fig. 7 is a side elevational View stone shown in Fig. 6.

Mo e specifically in accordance with the invention, alumina powder having mixed therewith between 0.1% and 0.3% of titania by weight as an essential ingredient (together with any colorant required by the particular crystal, such as a small quantity of chromium oxide for ruby or ferric oxide for blue sapphire) is placed in a basket screen ii within a hopper i3. Powder is periodically sifted out of the screen by intermittently striking an anvil 15, which projects from the top of the screen to the outside of the hopper, with a pivoted hammer ii actuated by a rotating cam it. The powder is picked up by a stream of gaseous oxygen supplied to hopper 23 through a conduit 21 leading from a pair oi supply conduits 22 and 24, after which the powder-laden oxygen passes down through a central pipe 23 forming a part of a vertical burner 55.

A fuel gas such as hydrogen is supplied through a conduit 27 to a casing 29 surrounding pipe 23 and passes down to the exit of the oxygen pipe where the hydrogen and oxygen mix together. The oxy-hydrogcn mixture laden with powder then leaves the blowpipe and burns as an intensely hot downwardly directed flame within a skirt 3! surrounding the upper end of a ver tical supporting pedestal 33 which carries an upstanding corundum seed crystal Seed crystal 35 is mounted so that the c-axis thereof makes the same angle with the axis along which the boule is to grow as is desired in the boule, for example degrees or zero degrees,

The powder drops through the flame and mulates in a molten condition on the seed crystal 35, the top of which has been melted in the flame. As the molten material accumulates, progressive crystallization to form a single crystal having the same crystallographic orientation as the seed crystal 35 is induced by gradually lowering the pedestal 33 vertically downwardly away from the flame to progressivel reduce the temperature of the melt to its freezing point. Any suitable lowering mechanism can be used, such as a screw 33 driving a traveling nut ll to which the pedestal is secured.

The shape and size of the crystal are controlled by maintaining proper size and temperature of the flame, proper powder dispensing rate, and proper crystal lowering rate, as is well known in the art. Ordinarily, the crystal is grown as a boule 43, shown in Fig. 2, of circular cross section having a lower end or foot 4-5 of small diameter which is broadened upwardly to the relatively larger diameter main body 4? by increasing the gas flows and the powder tapping rate.

When growing the main body ll of the boule, fluctuating thermal conditions are maintained around the crystal by alternately increasing and decreasing at frequent intervals the rate of oxygen feed to the oxy-hydrogen flame, as by fluctuations of between 1 and 3 cubic feet per hour. This advantageously is done by opening and closing a control valve 48 in oxygen supply conduit 24, while maintaining a constant flow of f the gem gas from the other oxygen supply conduit 22 and from hydrogen supply conduit 21. The crystal then grows as a series of thin upwardly convex longitudinally distributed tranverse layers, alternate ones of the layers deposited during the periods of low oxygen flow having titania across the full width thereof, and the intermediate layers deposited during the periods of high oxygen flow having titania only in annular peripheral zones. The layers are of the order of 0.004 inch thick when a boule is grown at about 0.5 inch per hour and the oxygen flow is fluctuated at intervals of 30 seconds.

In a specific example a 60-carat blue sapphire boule was grown with zero degrees c-axis orientation from 99.5% of alumina powder mixed with 0.2% of titania and 0.3% of ferric oxide by weight, using hydrogen at a rate of 106 cubic feet per hour and oxygen at rates of 3'7 and 38.5 cubic feet per hour alternately for periods about minute long.

In another specific example, a ruby boule one inch in diameter was grown with a 90 degrees c-axis orientation from 98.9% of alumina powder mixed with 0.1% of titania and 1% of chromium oxide by weight with a fluctuation of 2 cubic feet per hour in the oxygen flow at about A2 minute intervals.

Upon completing the growth of the boule 43 by the procedure described above it is either annealed, or is split vertically in a well-known manner. The annealed whole boule, or the halves of the split boule are then given an asteriating heat treatment to cause precipitation of a titanium compound, probably titania as rutile, from solid solution in the corundum crystal. This is accomplished by heat treating the crystal for more than two hours at a temperature within the range between 1100 and 1500 C. until a cloudy precipitate forms therein, the time of heating varying as an inverse function of the temperature. Other details of asteriating heat treatments are disclosed in U. S. Patent 2,488,507. When the asteriated boule is sectioned vertically and examined at high magnification it exhibits alternate layers 59 (see Fig. 3) containing a cloudy, silky precipitate of titanium compound across their entire width, and intermediate layers 5| containing a precipitate of titanium compound visible only in thin annular zones adjacent to their peripheries and substantially free from such a visible precipitate near the center of the boule. In a blue sapphire boule, the blue color also extends across the entire width of the alternate layers 49 but is confined to the peripheral zones of the intermediate layers 5|. A ruby boule, however, is red completely across all the layers.

A gem stone which exhibits a well-centered six-rayed star is then cut cabochon from the asteriated corundum boule. A gem stone of the most uniform and homogeneous appearance and color is best cut from a boule having a crystallographic c-axis orientation of zero degrees, i. e., the c-axis cc' of the boule (see Fig. 4) is parallel to the longitudinal vertical growth axis thereof. This is because, as shown in Fig. 4, a cabochon 53 can be cut from a boule =13 so that its convex crown 55 has approximately the same curvature as the curved layers 49 and 5! of the boule. In such a cabochon there is ostensibly continuous color distribution, and there is no detrimental banded appearance such as may sometimes be present in a cabochon which has been cut from a boule having a c-axis orientation greater than zero degrees (which requires cutting so that the crown 55 intersects the several layers at a large angle). When cutting blue sapphire from zero degrees boule, moreover, the thinnest portion of the cabochon near its edge comes from the part of the boule having the deepest blue color due to the existence of color in the adjoining peripheral zones of layers 49 and 5|, thus providing uniform color across the crown of the cabochon.

Commercially satisfactory synthetic ruby and sapphire cabochons may also be cut from a degree boule #53, shown in Fig. 5, in such a way that the thin curved layers 49' and 5! extend approximately parallel to the geometric axis of the cabochon 53' and the approximately flat base 56 of the cabochon cuts the several layers 49 and til at a large angle of about 90 degrees. This procedure is especially advantageous for cutting a cabochon having one diameter greater than the diameter of the boule.

Figs. 6 and '7 show the appearance of the ire-- proved gem stones of the invention wherein the six rays of the star 5'! extend full length down the sides of the cabochon 53 to the base 55 and are of equal intensity, thus adding greatly to the pleasing appearance of the gem stones.

The principles of the present invention have been described in detail above by way of illustration only. It will be evident to those skilled in the art that fluctuating thermal conditions around the growin crystal can be maintained in other ways than by fluctuating the oxygen flow, for example by such expedients as fluctuating the hydrogen flow rate; fluctuating both the oxyge and hydrogen flow rates; fluctuating the rate of powder feed; or regularly changing the position of the growing crystal within the furnace as by alternately lowering and raising the crystal.

What is claimed is:

l. A process for growing a corundum monccrystal containing titania wherein such ti .n t apparently is distributed throughout said crysun, said process comprising passing powdered constituents of said crystal comprising a mixture of alumina and titania through a flame to fuse said constituents; forming a corundum crystal of i creasing size by accumulating and crystalli said fused constituents on a support; and gro. ing said crystal as a series of thin longitudin distributed upwardly convex transverse lay "s, alternate ones of said layers having ti maria across the full, width thereof, the rest of said layers having titania concentrated adjacent peripher thereof but being substantially free therefrom adjacent the center thereof, by maintaining fluctuating thermal conditions around said corundum crystal during said. accumulating and crystailizing.

2. A process for growing a corundum crystal in accordance with claim 1 wherein said crystal is grown on a corundum seed with a c-axis orientation of about 99 degrees.

3. A process for growing a ccrunduzn crystal in accordance with claim 1 wherein said crystai is grown on a corundum seed with a c-axis orien tation of about zero degrees.

4. A process for growing a corunduxn crystal containing titania wherein such titania ently is distributed throughout said crystal, process comprising passing powdered constituents of said crystal comprising a mixture of: alumina and titania through a flame to {use said constituents; forming a corunduin crystal of increasing size by accumulating and crystallizing said fused constituents on a support; and, during said accumulating and crystallizing, alternately increasing and decreasing the rate of heat supply to said corundum crystal and thereby growing said crystal as a series of thin longitudinally distributed transverse layers, alternate ones of said layers having titania across the full width thereof, the rest of said layers having titania concentrated adjacent the periphery thereof but being substantially free from titania adjacent the center thereof.

5. A process for growing a corundum crystal containing titania wherein such titania apparently is distributed throughout said crystal, said process comprising passing powdered constituents of said crystal comprising a mixture of alumina and titania through a flame to fuse said constituents; forming a corundum crystal of increasing size by accumulating and crystallizing said fused constituents on a support; and, during said accumulating and crystallizing, alternately increasing and decreasing the heating intensity of said flame at frequent intervals to grow said crystal as a series of thin longitudinally distributed transverse layers, alternate ones of said layers having titania across the full width thereof, the rest of said layers having titania concentrated adjacent the periphery thereof but being substantially free from titania adjacent the center thereof.

6. A process for growing a corundum crystal containing titania wherein such titania apparently is distributed throughout said crystal, said process comprising passing powdered constituents of said crystal comprising a mixture of alumina and titania through an oxy-fuel gas flame to fuse said constituents; accumulating and crystallizing said fused constituents on a support aligned axially with said flame to form a corundum crystal of increasing size; and, during said accumulating and crystallizing, alternately increasing and decreasing at frequent intervals the rate of oxygen feed to said flame and growing said crystal as a series of thin longitudinally distributed transverse layers, alternate ones of said layers having titania across the full width thereof, the rest of said layers having titania concentrated adjacent the periphery thereof.

7. A process in accordance with claim 6 wherein the rate of oxygen feed is alternately increased and decreased by between 1 and 3 cubic feet per hour.

8. A process in accordance with claim '2 wherein the rate of oxygen feed is alternately increased and decreased at about /2 minute intervals.

9. A process in accordance with claim 8 wherein said titania comprises about 0.1 to 0.3% of said mixture.

10. A process in accordance with claim 6 wherein the rate of oxygen feed is alternately increased and decreased at about A minute intervals.

11. A process for making an asteriated corundum crystal wherein an asteriating precipitate apparently is distributed throughout said crystal, said process comprising passing powdered constituents of said crystal comprising a mixture of alumina with titania through a flame to fuse said constituents; accumulating and crystallizing said fused constituents on a support to form a corundum crystal of increasing size; maintaining fluctuating thermal conditions around said crystal during said accumulating and crystallizing thereby growing said crystal as a series of thin longitudinally distributed transverse layers, alternate ones of said layers having titania across the full width thereof, the rest of said layers having titania concentrated adjacent the periphery thereof but being relatively free from titania near the center thereof; and thereafter heating said crystal in the temperature range between 1100" and 1500 C. for a period of time greater than 2 hours until a cloudy asteriating precipitate forms therein.

12. A process for producing an asteriated corundum gem stone wherein an asteriating precipitate apparently is distributed throughout, said process comprising passing powdered constituents of said crystal comprising a mixture of alumina with titania through a flame to fuse said constituents; accumulating and crystallizing said constituents on a support comprising a corundum seed crystal having a c-axis. orientation of about zero degrees to form a crystal of increasing size; during said accumulating and crystallizing maintaining fluctuating thermal conditions around said crystal and growing said crystal as a plurality of thin longitudinally distributed upwardly convex transverse layers, alternate ones of said layers having titania across the full width thereof, the rest of said layers having titania concentrated adjacent the periphery thereof but being substantially free from titania near the center thereof; annealing said crystal to prevent split ting thereof; thereafter heating said crystal in the temperature range between 1100 and G C. for a period of time greater than 2 hours until a cloudy precipitate forms therein; and then cutting a gem stone cabochon from said crystal with its crown approximately coinciding with the curvature of said layers and with the tangent to said crown at the apex thereof normal to the c-axis of said crystal.

13. A method for producing a synthetic star corundum gem stone comprising providing a synthetic corundum crystal as an asteriated whole boule having a c-axis orientation of zero degrees, said boule comprising a series of thin longitudinally distributed upwardly convex transverse layers symmetrical with respect to the longitudinal axis of said boule, alternate ones of said layers having a compound of titanium precipitated therein across the full width thereof, the rest of said layers having a compound of titanium precipitated therein adjacent the periphery thereof but being substantially free from such compound near the center thereof; and cutting a gem stone cabochon from said boule with its crown approximately coinciding with the curvature of said layers and with the tangent to crown at the apex thereof normal to the c-axis of said crystal.

14. A single crystal of synthetic corundum comprising a series of thin curved layers, alternate ones of said layers having a compound of titanium distributed throughout, the rest of said layers having a compound of titanium adjacent the periphery thereof but being substantially free from said compound near the center thereof.

15. A monocrystalline boule of synthetic corundum having a longitudinal axis, said boule comprising a series of longitudinally distributed transversely extending thin curved layers arranged symmetrically with respect to said axis, alternate ones of said layers having a compound of titanium distributed across the full width thereof, the rest of said layers having a compound of titanium adjacent the periphery thereof but being substantially free of said compound near the center thereof.

16. An asteriated single crystal of synthetic corundum comprising a series of thin curved layers, alternate ones of said layers having a precipitate of a compound of titanium distributed throughout, the rest of said layers having such a precipitate adjacent the periphery thereof but being substantially free of such a precipitate near the center thereof.

17. An asteriated monocrystalline boule of synthetic, corundum having a longitudinal axis, said boule comprising a series of longitudinally distributed transversely extending thin curved layers arranged symmetrically with respect to said axis, alternate ones of said layers having a precipitate of a compound of titanium distributed across the full width thereof, the rest of said layers having a precipitate of such a compound adjacent the periphery thereof but being sub stantially free of such a precipitate near the center thereof.

18. An asteriated monocrystalline boule of synthetic corundum in accordance with claim 17 wherein the c-axis thereof is substantially parallel to said longitudinal axis.

19. An asteriated monocrystalline cabochon of synthetic corundum having a geometric axis and a convex crown arranged symmetrically with respect to said geometric axis, said cabochon comprising a series of axially distributed thin curved layers extending transversely of and arranged symmetrically with respect to said geometric axis, the curvature of said crown being approximately the same as said layers, alternate ones of said layers having a precipitate of a compound of titanium distributed across the full Width thereof, the rest of said layers having a precipitate of such a compound adjacent the periphery thereof but being substantially free of such a precipitate near the center thereof, the c-axis of said cabochon extending parallel to said geometric axis.

20. An asteriated monocrystalline cabochon of synthetic corundum having a geometric axis and a convex crown arranged symmetrically with respect to said geometric axis, said cabochon comprising a series of thin curved layers extending approximately parallel to said geometric axis, alternate ones of said layers having a precipitate of a compound of titanium distributed across the full width thereof, the rest of said layers having a precipitate of such a compound adjacent the periphery thereof but being substantially free of such a precipitate near the center thereof, the c-axis of said carbochon extending parallel to said geometric axis.

21. A process for growing a corundum crystal containing titania wherein such titania apparently is distributed throughout said crystal, said process comprising passing powdered constituents of said crystal comprising a mixture of alumine and titania through an oxygen gas-fuel gas flame to fuse said constituents, said titania comprising about 0.1 to 0.3% of said mixture; accumulating and crystallizing said fused constituents on a support aligned axially with said flame to form a corundum crystal of increasing size; and, during said accumulating and crystallizing, alternately increasing and decreasing at frequent intervals the rate of feed of at least one of said gases to said flame and growing said crystal as a series of thin longitudinally distributed transverse layers, alternate ones of said layers having titania across the full width thereof, the rest of said layers having titania concentrated adjacent the periphery thereof.

References (Jited in the file of this patent OTHER REFERENCES Synthetic Corundum for Jewel Bearings,

Sandmeier, 72 Jour. Inst. of Elec. Eng, London 1933, pages 506, 507, 509 (Dr. Rayners queries) 513 (response) and Plates 3 and 4. 

1. A PROCESS FOR GROWING A CORUNDUM MONOCRYSTAL CONTAINING TITANIA WHEREIN SUCH TITANIA APPARENTLY IS DISTRIBUTED THROUGHOUT SAID CRYSTAL, SAID PROCESS COMPRRISING PASSING POWDERED CONSTITUENTS OF SAID CRYSTALS COMPRISING A MIXTURE OF ALUMINA AND TITANIA THROUGH A FLAME TO FUSE SAID CONSTITUENTS; FORMING A CORUNDUM CRYSTALS OF INCREASING SIZE BY ACCUMULATING AND CRYSTALLIZING SAID FUSED CONSTITUENTS ON A SUPPORT; AND GROWING SAID CRYSTAL AS A SERIES OF THIN LONGITUDINALLY DISTRIBUTED UPWARDLY CONVEX TRANSVERSE LAYERS, ALTERNATE ONES OF SAID LAYERS HAVING SAID TITANIA ACROSS THE FULL WIDTH THEREOF, THE REST OF SAID LAYERS HAVING TITANIA CONCENTRATED ADJACENT THE PERIPHERY THEREOF BUT BEING SUBSTANTIALLY FREE 